It is known to transmit protected multimedia content to terminals that are connected to the same access-rights server using a long-distance information transmission network. In these known methods the terminal sends, to the access-rights server, an access-rights request to receive the access rights authorizing it to access the multimedia content. Then, in response to the access-rights request, the access-rights server adds the terminal's identifier to a broadcast list. The access-rights server then broadcasts, at predetermined intervals, each new access right to the multimedia content only to those terminals whose identifier appears in the broadcast list and without waiting for these terminals to have requested this new access right by sending a new access rights request.
In response to having received each access right, the terminal accesses the multimedia content that has been transmitted. In the absence of having received the access right, the terminal prevents access to the multimedia content that has been transmitted.
At any given time, the terminal switches, independently of the other terminals from a ready state to an occupied state. During the ready state, the terminal is able to send, by means of a point-to-point connection, the access rights request and to receive the requested access right. During the occupied state, the terminal is unable to do either.
In the case of the protected multimedia content by a conditional-access system (“CAS”), an entitlement-management message EMM contains an access right DAi. This access right DAi typically contains a cryptogram KS*(Ki) of an operating key KS that was encrypted with a private key Ki of a terminal Ti. This access right DAi is specific to the terminal Ti since only this terminal Ti has the private key K and is thus able to decrypt the cryptogram KS*(Ki) in order to obtain the key KS allowing it to access the protected multimedia content. In particular, even if other terminals receive the access right DAi, those other terminals cannot use it. This is because those other terminals lack the private key Ki.
The foregoing method strengthens the security of the method for transmitting protected multimedia content. On the other hand, the access-rights server needs to prepare and send as many specific access rights DAi as there are different terminals wishing to access this protected multimedia content. Given that the number of terminals may be very large, such as more than 10,000 or 1,000,000, this consumes a sizeable amount of bandwidth in the transmission network.
To ameliorate this difficulty, it is known to distribute the terminals into several groups and to assign, to each group, a specific time slot. The access-rights server then transmits the access rights DAi for a group of terminals only during the time slot associated with that group. This reduces the bandwidth needed to send the access rights DAi to the terminals.
A difficulty with the foregoing solution is the need to form and manage groups of terminals.